Campbell Manufacturing RF415 ユーザーズマニュアル
Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors
H-4
Antenna Gain
Antenna gain is specified either in dBi (decibels of gain relative to an isotropic
radiator) or in dBd (decibels of gain relative to a dipole). The relationship is:
radiator) or in dBd (decibels of gain relative to a dipole). The relationship is:
dBi = dBd + 2.15
Some antennas that are FCC approved for use with the RF400 series are:
Mfg.
Antenna Type
Band
Model
CSI Item #
dBd
Gain
Gain
dBi
Gain
Gain
Size
Astron
Omni (1/2 wave)
900 MHz
AXH900 RP SMA R
14204
0
2.15
6.75”
Antenex
Collinear
900 MHz
FG9023
14221
3
5.15
24”
MaxRad
Yagi
900 MHz
BMOY8905
14201
9
11.15
21.4”
LINX
Omni (1/2 wave)
2.4 GHz
ANT-2.4-CW-RCT-RP
16005
0
2.15
4.5”
MaxRad
Enclosed Yagi
2.4 GHz
WISP24015PTNF
16755
13
15.1
17”
Receiver Sensitivity
Receiver sensitivity is usually specified in dBm for a specific bit error rate
(BER). The transceiver module used in the RF400 (either 900 MHz radio) is
specified at –110 dBm at ~10
(BER). The transceiver module used in the RF400 (either 900 MHz radio) is
specified at –110 dBm at ~10
-4
raw BER.
If the received signal strength is greater than the receiver sensitivity, a link can
be established. Any excess signal strength above the receiver sensitivity is
“link margin”, and is a very good thing; a minimum of 6 dB of link margin
should be sought.
be established. Any excess signal strength above the receiver sensitivity is
“link margin”, and is a very good thing; a minimum of 6 dB of link margin
should be sought.
Path Loss
We have combined in this section the normal “free space” path loss (only seen
in mountaintop to mountaintop scenarios) with loss due to ground reflections,
diffraction, leaf/forest absorption, etc. It is all loss!
in mountaintop to mountaintop scenarios) with loss due to ground reflections,
diffraction, leaf/forest absorption, etc. It is all loss!
A starting point is the “free space” path loss. Here are two equations for this:
Lp (dB) = 32.4 + 20 x log( f ) + 20 x log ( d ) dB
(f in MHz, d in km)
Lp (dB) = 36.6 + 20 x log( f ) + 20 x log ( d ) dB
(f in MHz, d in miles)
Here is a table showing the free space path loss (in dB). Note the effect of
frequency.
frequency.
Frequency
Distance
1 mi.
2 mi. 4 mi.
8 mi.
10 mi.
16 mi.
22 mi.
26 mi.
30 mi.
400 MHz
89
95
101
107
109
113
115
117
118
915 MHz
96
102
108
114
116
120
123
124
125
2.4 GHz
104
110
116
122
124
128
131
133
134
Notice the relationship between path loss and distance: each time you double
the distance, you lose 6 dB of signal under free space conditions. Or, put
another way, if you add 6 dB of gain (for example with 6 dB of additional
antenna gain, or 6 dB less cable loss), you can double the distance for free
space conditions.
the distance, you lose 6 dB of signal under free space conditions. Or, put
another way, if you add 6 dB of gain (for example with 6 dB of additional
antenna gain, or 6 dB less cable loss), you can double the distance for free
space conditions.